Horse-fly is the most widely used English common name for members of the family Tabanidae. Apart from the common name "horse-flies", broad categories of biting, bloodsucking Tabanidae are variously known as breeze flies,[1]cleggs, klegs, or clags, deer flies, gadflies, or zimbs. In some areas of Canada, they also are known as bull dog flies, and in Newfoundland and Labrador as stouts. In Australia some species are known as "March flies", a name that in other English-speaking countries refers to the non-bloodsucking Bibionidae.

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The Tabanidae are true flies and members of the insectorder Diptera. Species of Tabanidae that habitually attack humans and livestock are widely regarded as pests because of the bites that females of most species inflict, and the diseases and parasites that some species transmit. The various species of Tabanidae range from medium-sized to very large in size. Some species, such as deer flies and the Australian March flies, are known for being extremely noisy during flight, though clegs, for example, fly quietly and bite with little warning. Tabanids are extremely fast and agile fliers. They have been observed to perform aerial maneuvers otherwise performed by fighter jets, such as the Immelman turn.[2]

In spite of their roles as pests, Tabanidae also are important pollinators of some flowers. In particular, several South African species have spectacularly long proboscides adapted to the extraction of nectar from flowers with long, narrow corolla tubes, such as Lapeirousia[3] and some Pelargonium.[4] Tabanidae occur worldwide, being absent only on some remote oceanic islands and at extreme northern and southern latitudes.

The genus Zophina is of uncertain placement, though it has been classified among the Pangoniinae. Two well-known genera are the common horse flies, genusTabanusLinnaeus, 1758 and the deer flies, genus ChrysopsMeigen, 1802 are also known as banded horse flies because of their coloring. Both genera give their names to subfamilies. The "Blue Tail Fly" in the eponymous song was probably a tabanid common to the southeastern United States.

Adult horse flies feed on nectar and sometimes pollen. Females of most species are anautogenous, meaning they require a blood meal before they are able to reproduce effectively, if at all. Much like male mosquitoes, male Tabanidae are not ectoparasitic and lack the mouth parts (mandibles) that the females use in drawing the blood on which they feed. Most female horse flies feed on mammalian blood, but some species are known to feed on birds or reptiles. Some are said to attack amphibians as well.

Larval horse flies are predators of small invertebrates in moist environments, such as in mud on the edges of bodies of water, in damp soil, under stones, or in rotting logs.

Horse fly bites are painful, the bites of large specimens especially so. Most short tongued (short proboscid) species of horse flies use their knife-like mandibles to rip and/or slice flesh apart.

Horse fly bites are more immediately painful than those of its mosquito counterparts, although it still aims to escape before its victim responds. The flies are very agile and adept at flying. Their bites may become itchy, sometimes causing a large swelling afterward if not treated quickly.

They are often not deterred by attempts at swatting them away, and will generally persist in attacking until they secure their quarry or are killed. Many often will give chase to their intended targets for a short time.

Mating is done in swarms, generally at landmarks such as hilltops. The season and time of day, and type of landmark, used for mating swarms is specific to particular species.[5] Eggs are laid on stones or vegetation usually close to water. On hatching, the larvae fall into water or moist earth, feeding voraciously on invertebrates, such as snails, earthworms and other insects.

Blood loss is a common problem in some animals when large flies are abundant. Some animals have been known to lose up to 300 millilitres of blood in a single day to tabanid flies, a loss which can weaken or even kill them. There are anecdotal reports of horse-fly bites leading to fatal anaphylaxis in humans, an extremely rare occurrence.[6]